An ignition coil 91 shown in FIG. 36 is used for energizing a spark plug, thereby generating spark in the spark plug. The ignition coil 91 is received in a plughole of an internal combustion engine of a vehicle, or the like. The ignition coil 91 includes a cylindrical portion 92 that accommodates a primary coil 921, a secondary coil 922, and a center core 94, which are coaxially arranged. The cylindrical portion 92 has a tip end 901, in which a plug holder 971 is formed. The cylindrical portion 92 has a rear end 902, to which an igniter 972 is provided for supplying electricity to the primary coil 921. As shown in FIG. 37, the secondary coil 922 is wound around a secondary spool 93, which is formed of resin to be in a cylindrical shape. The secondary spool 93 has a tapered inner surface 931, which is formed as a matter of convenience in a forming process of the secondary spool 93. The tapered inner surface 931 has the inner diameter that increases as being distant from the tip end 930 of the secondary spool 93. The center core 94 is arranged inside of the secondary spool 93. The center core 94 is constructed of multiple silicon steel plates, which are stacked in the radial direction of the center core 94. The center core 94 has the outer diameter that is axially constant. The outer diameter of the center core 94 corresponds to the inner diameter smallest of the secondary spool 93. The center core 94 and the secondary spool 93 defines a gap therebetween. This gap becomes large, as being distant from the tip end 930 of the secondary spool 93.
The igniter 972 inputs an ignition timing signal from an electronic control unit (ECU) of the engine, so that the igniter 972 supplies electricity to the primary coil 921. Thus, the primary coil 921 generates magnetic flux passing through the center core 94, thereby causing an interlinkage with respect to the secondary coil 922. The secondary coil 922 generates induced electromotive force by electromagnetic induction, thereby generating spark in the sparkplug mounted to the plug holder 971. Magnetic flux generated using the primary coil 921 passes through the center core 94, thereby being enhanced.
According to JP-A-10-41152, the outer diameter of the center core is increased for enhancing induced electromotive force generated in the secondary coil. Conventionally, when the outer diameter of the center core is increased, the ignition coil is jumboized. Consequently, the inner diameter of the plughole of the engine needs to be increased. However, it is difficult to increase the outer diameter of the plughole, in a downsized engine.
In addition, according to JP-A-8-167518, a center core having an enhanced magnetic property is disclosed. However, it is still demanded to produce a center core, which is capable of producing high power, and to restrict manufacturing cost of the center core from increasing.
In recent years, a high power ignition coil having a downsized structure is demanded. An ignition coil has a center core and an outer core, which are separated from each other. In this structure, the ignition coil has an open magnetic circuit, in which magnetic efficiency may decrease due to leakage of magnetic flux. Particularly, when the center core and the outer core interpose an air space therebetween, leakage of magnetic flux in the air space becomes large, because of a large magnetic resistance in the air space. According to JP-A-11-87157, an ignition coil has a structure, in which magnetic flux is restricted from leaking. However, even in this structure, it is difficult to restrict magnetic flux from leaking.